1. Field of the Invention
The invention relates to the field of electrically controllable displays and more particularly to the field of electrochromic displays.
2. Background of the Invention
There are many uses for electrically controllable display devices. A number of such devices have been in commercial use for some time. These display devices include liquid crystal displays, light emitting diode displays, plasma displays and so on. Light emitting diode and plasma display panels both suffer from the fact that they are active, light emissive devices which require substantial power for their operation. In addition, it is difficult, if not impossible, to fabricate light emitting diode displays in a manner which renders them easily distinguishable under bright ambient illumination. Liquid crystal displays suffer from the disadvantage that they are operative only over a limited temperature range and have substantially no memory within the liquid crystal material. Further, the visibility of many liquid crystal displays decreases as the viewer moves a few degrees off axis.
Electrochromic displays have been developed which display information through a change in the color of portions in the display via electrodeposition of an active material to achieve a color change. Generally, this color change is from white to a color such as blue. Because of their electrodeposition mechanism, such displays require substantial power and time to write or erase displayed information. The quantities of power required are undesirably large, especially for battery operation, and the time required to change displayed information makes such materials unacceptable for many display applications. None of these displays display more than a single color against a background. This limits the versatility of such displays since the color of a character cannot be used to convey information.
Rare earth diphthalocyanines are known from prior publications to have electrochromic properties in which the color of the diphthalocyanine can change over a period of about eight seconds upon application of a potential difference across an electrochemical cell having a diphthalocyanine film on one of the electrodes. P. N. Moskalev and I. S. Kirin, "Effect of the Electrode Potential on the Absorption Spectrum of a Rare-Earth Diphthalocyanine Layer," Opt. i Spektrosk, 29, 414 (1970) and P. N. Moskalev and I. S. Kirin, "The Electrochromism of Lanthanide Diphthalocyanines" Russian J. Phys. Chem., 46, 1019 (1972). The diphthalocyanine does not require large amounts of power to change color, but the long period required for the color to change makes known diphthalocyanine performance characteristics unacceptable when measured against display requirements.
Consequently, a display device is needed which can be rapidly switched, produces several colors, retains its memory with the power turned off, can be easily read under a wide range of ambient light conditions and viewing angles an does not require large quantities of power for its operation.